My invention relates generally to a system for monitoring the structural integrity of a bonded joint and, more specifically, to a system for continuous in-service monitoring of bond degradation corresponding to measured changes in the dielectric properties of the bonding material.
Practitioners in the art of advanced composites have monitored the curing of a composite matrix or laminate part by measuring changes in the dielectric properties of the composite matrix resin such as resistance or capacitance. As the resin cures, changes in the dielectric measurements are used to infer resin viscosity. Active feedback control systems maintain optimal temperature and pressure in autoclave cure cycles in response to changes in resin viscosity.
In these autoclave control systems, small integrated circuit sensors such as the EUMETRIC.TM. sensor manufactured by Micromet Instruments of Cambridge, Mass. may be attached to the laminate or embedded in it. The sensors are connected by cables in the autoclave to a microdielectrometer, which is a sensitive instrument for measuring small changes in the dielectric properties of the laminate.
Although microdielectrometers and dielectric sensors are well-known in the art, resin-curing monitors are not designed to detect dielectric changes corresponding to decreases in bond strength; they only monitor dielectric changes corresponding to increases in bond strength such as would be experienced during an autoclave curing cycle.
The dielectric properties of the laminate part continue to change long after it is fabricated as it absorbs moisture from the environment. This change, however, is in the reverse direction of that experienced during resin-curing. Moisture absorption results in degradation of structural strength and, eventually, delamination of the part.
Structural members may be bonded using an adhesive resin, which degrades and weakens over time due to moisture absorption in the same manner as in composite laminates. High moisture levels lower the strength of the adhesive, eventually causing failure of the joint.
Critical bonded joints of a structure such as an aircraft are often inaccessible and uninspectable. It is an object of my invention to provide an indication of environmental degradation of bonded joints in structures that are not visually inspectable. It is a further object of my invention to detect not only failures but also trends in the dielectric properties of bonded joints that can be used to estimate the remaining service life of the bonded structure. These problems and deficiencies are clearly felt in the art and are solved by the present invention in the manner described below.